1. Field of the Invention
The present invention relates to a line test trunk interface circuit connected between a test desk and an electronic switching center and more particularly to improvements to said circuit providing for low power consumption via the use of low power rated components while maintaining the integrity of the operation of the line test trunk circuit and preventing false operation of the circuit.
(2) Description of the Prior Art
When testing central office equipment associated with a subscriber line to verify such problems as lack of dial tone, wrong number on called origination, etc., a test desk facility is utilizied to perform functional testing of the equipment. Various equipment associated with a subscriber line connected to electronic switching center can be routined by a craftsman at a centrally located test desk. A test desk facility is interfaced with an electronic switching center via a circuit called a line test trunk which is similar to an incoming trunk circuit with additional testing cability. The functions and connections of one such test desk is shown in an article contained in the GTE Automatic Electric Technical Journal by Daniel J. Mazzola entitled "NO. 1 EAX in Taiwan", Vol. 15-No. 9, September, 1977, p. 378.
A line test trunk circuit connects a test desk to a trunk matrix and a selector matrix of an electronic switching center. In addition, supervisory signals are supplied to the central processing unit of an electronic switching center via a maintenance and control center. The electronic switching center referred to above is described in U.S. Pat. No. 3,835,260, "Communication Switching System with Marker, Register and Other Subsystems Cordinated by a Stored Program Central Processor", issued on Sept. 10, 1974, to K. E. Prescher et al. The functions and interface connections of the Maintenance and Control Center as mention above are described in GTE Automatic Electric Technical Journal, in an article entitled "NO. 1 EAX (Electronic Automatic Exchange) Maintenance and Control Center", by T. W. Crosley, et al, Vol. 13-No. 4, October, 1972, p. 163.
Referring to prior art FIG. 2, the test desk connection to the line test trunk circuit via a cable is shown. When relay K4 operates a supervisory signal is transmitted to the central processing unit of the electronic switching center via the maintenance and control center not shown. It is intended that the operation of relay K4 be a function of the operation of switch BCO by the craftsman. Contact K10 is operated via a relay not shown. When the craftsman inserts a test cord into a jack contained on the test desk current flows from the -50 volt supply through resistor R3, switch BCO, via lead S over cable (shown as resistance RC), break contact K4', relay K4, make contact K10 to ground. The circuit in this state depicts the original interface provided by the line test trunk to the test desk facility. During operation of the above-mentioned electronic switching center in Taiwan, it was found that relay K4 was operating in response to the insertion of the test cord into the jack located on the test desk. This operation of relay K4 caused false indication to the central processing unit associated with the switching center.
One solution to this problem was proposed as shown in prior art FIG. 3. A resistor R23 is connected in parallel with relay K4 and to the common connection of resistor R13 and break contact K4'. The operate and hold currents for relay K4 are fixed at 17.4 ma and 9.0 ma, respectively. The value of shunting resistor R23 was determined to be 249 ohms at 5 watts requiring a flameproof resistor. In order to maintain the holding current of 9.0 ma resistor R13 was required to be changed from 825 ohms at 1 watt to 300 ohms at 5 watts. This results in a high current drain of 125.2 ma and excessive power consumption of 2.76 watts by resistor R23.
Therefore, it is an object at the present invention to provide a low power consumption circuit for interfacing a test desk to an electronic switching system. In addition, it is an object of the present invention to eliminate false operation of said relay providing a false signal to the switching system requesting that a path be established between the line to be tested and the common control switching equipment of the switching center. Furthermore, it is an object of the present invention to provide the above mentioned objectives without altering the test desk or other common control equipment included in the switching center.